Process for liquefying natural gas employing a multicomponent refrigerant for obtaining low temperature cooling

ABSTRACT

Refrigeration for liquefying natural gas is provided by a closed cycle refrigeration system employing a multicomponent refrigerant. A stream of multicomponent refrigerant flowing in the closed refrigeration system is compressed and then successively fractionated by partial condensation in a plurality of steps to provide condensates at progressively decreasing temperature levels. The condensates are separated and introduced under reduced pressure into a common zone in heat exchange with the natural gas and vaporization of the condensates. A stream of multicomponent refrigerant including the resulting vapor from the condensates is withdrawn from the zone for recycle. The multicomponent refrigerant includes a mixture of components consisting essentially of nitrogen and a plurality of hydrocarbons. In one presently preferred variant, the refrigerant is a mixture of nitrogen and more than three hydrocarbons having molecular weights between the molecular weight of methane and the molecular weight of hexane. Two of the hydrocarbons are methane and ethane. Ethane is the component of greatest percentage, methane is the component of second greatest percentage, and nitrogen is present in a percentage substantially less than that of methane.

Elite tes Gaumer, Jr. et al.

atent [54] PROCESS FOR LIQUEFYHNG NATURAL GAS EMPLOYING A MULTTCOMPONENTREFRHGERANT FOR OBTAINING LOW TEMPERATURE COOLING [72] Inventors: Lee S.Gaumer, Jr.; Charles L. Newton,

both of do Air Products & Chemicals, lnc., P.O. Box 538, Allentown, Pa.18105 [22] Filed: June 25,1970

[21] Appl.No.: 49,622

Related U.S. Application Data [63] Continuation-in-part of Ser. No.882,781, Dec. 22, 1969, and a continuation-in-part of 3,571, Jan. 9,1970, abandoned.

[52] US. Cl ..62/9, 62/11, 62/40 [51] Int. Cl ..F25j 1/00, F25j 3/06[58] Field of Search ..62/9, 23, 24, 27, 28, 40

[56] References Cited UNITED STATES PATENTS 3,364,685 l/1968 Perret..62/40 NATURAL GAS Primary Examiner-Norman Yudkoff Assistant ExaminerA.Purcell Attorney-Shanley and ONeil [5 7] ABSTRACT Refrigeration forliquefying natural gas is provided by a closed cycle refrigerationsystem employing a multicomponent refrigerant. A stream ofmulticomponent refrigerant flowing in the closed refrigeration system iscompressed and then successively fractionated by partial condensation ina plurality of steps to provide condensates at progressively decreasingtemperature levels. The condensates are separated and introduced underreduced pressure into a common zone in heat exchange with the naturalgas and vaporization of the condensates. A stream of multicomponentrefrigerant including the resulting vapor from the condensates iswithdrawn from the zone for recycle. The multicomponent refrigerantincludes a mixture of components consisting essentially of nitrogen anda plurality of hydrocarbons. In one presently preferred variant, therefrigerant is a mixture of nitrogen and more than three hydrocarbonshaving molecular weights between the molecular weight of methane and themolecular weight of hexane. Two of the hydrocarbons are methane andethane. Ethane is the component of greatest percentage, methane is thecom ponent of second greatest percentage, and nitrogen is present in apercentage substantially less than that of methane.

8 Claims, 2 Drawing Figures MULTl-COMPONENT REFRIGERANT PATENTEUFEB 29m2 SHEET 1 BF 2 INVENTORS LEE 8. GAUMER,JR NEWTON CHARLES L.

REFRIGERANT MULTI-COMPONENT ATTORNEYS PROCESS FOR LIQUEFYING NATURAL GASEMPLOYING A MULTICOMPONENT REFRIGERANT FOR OBTAINING LOW TEMPERATURECOOLING CROSS-REFERENCE TO RELATED APPLICATIONS The preset applicationis a continuation-in-part of out copending application Ser. No. 882,781,filed Dec. 22, 1969 for Liquefaction of Natural Gas, and out copendingapplication Ser. No. 3,571, filed Jan. 9, 1970 for MulticomponentRefrigerator for Obtaining Low Temperature Cooling. Application Ser. No.882,781 is a continuation of our abandoned application Ser. No. 722,136,filed Apr. 17, 1968 for Liquefaction of Natural Gas. Application Ser.No. 3,571 now abandoned is a continuation of our abandoned applicationSer. No. 659,988, filed Aug. 11, 1967 for Multicomponent Refrigerant ForObtaining Low Temperature Cooling. Abandoned applications Ser. No.659,988 and 722,135 were a continuation-in-part and a continuation,respectively, of our abandoned application Ser. No. 468,008, filed June29, 1965 for Liquefaction of Natural Gas.

BACKGROUND OF THE INVENTION The present invention broadly relates to theliquefaction of low boiling point gases and, more particularly, to animproved method and apparatus particularly designed for liquefyingnatural gas with a substantial reduction in the cost of the liquefactionfacility as compared to previous liquefaction cycles of the cascade typewherein the process steam is heat exchanged with different refrigerantscirculated in independent closed loops. In one of its more specificvariants, the invention relates to an improved process for liquefyingnatural gas wherein refrigeration is provided by a closed cyclerefrigeration system employing a multicomponent refrigerant. Theinvention further relates to improvements in refrigerants of themulticomponent type, i.e., a mixture of component gases of differentboiling points.

Refrigeration systems employing multicomponent refrigerants are providedby the prior art. U.S. Pat. No. 2,041,725 of Podbielniak discloses arefrigeration system for obtaining a source of refrigeration at lowtemperature in which a multicomponent refrigerant is partially condensedat a plurality of decreasing temperature levels. The condensate at thelowest temperature provides the source of refrigeration and thecondensates under reduced pressure are successively passed incountercurrent heat interchange with the refrigerant to effect thepartial condensation steps. The Podbielniak patent teaches that it isonly necessary that the refrigerant comprise a series of constituentshaving a range of condensation temperature such as mixtures ofhydrocarbon gases including natural gas, refinery gas, coal gas andwater gas, as well as chlorinated or fluorinated hydrocarbons. A processof A. P. Kleemenlro (Progress in Refrigeration Science and Technology,Volume I, pages 34-39, published in 1960 by Pergamon Press, Library ofCongress Card No. 60-16886) embodies the multicomponent refrigerationteaching of the Podbielniak patent in a cycle for liquefying air ornatural gas wherein the feed gas is passed continuously incountercurrent heat interchange with the condensates and their resultantvapors of a multicomponent refrigerant which is flowed in a closedcycle. Kleemenko recognizes that the efficiency of the process willdepend upon the irreversibility of the heat interchange between therefrigerant and the fluid being cooled and teaches that smalltemperature differences between the refrigerant and the fluid may beobtained throughout the heat interchange by utilizing a multicomponentrefrigerant. For that purpose, Kleemenko discloses an optimumrefrigerant comprising a mixture of hydrocarbon gases consisting of 65percent methane, 20 percent propane and percent butane while alsodisclosing mixtures of methane and propane and mixtures of methane,ethane and propane. It is also known in the art to employ natural gas asa multicomponent refrigerant in a process for liquefying natural gas.

Although a process embodying the Kleemenko teachings provides improvedefficiency, it has been determined that potential advantages ofmulticomponent refrigeration systems in terms of overall efficiencycannot be achieved by utilizing multicomponent refrigerants ofcompositions taught by the prior art.

Accordingly, it is an object of the present invention to provide amulticomponent refrigerant of novel composition as disclosed hereinafterwhich makes it possible to obtain fully the potential advantages of themulticomponent refrigeration process in terms of overall efficiency andcost of capital equipment.

It is a further object to provide an improved process for liquefyingnatural gas wherein refrigeration is provided by a closed cyclerefrigeration system employing a multicomponent refrigerant comprisingnitrogen and a plurality of hydrocarbons including methane and ethane,wherein the ethane is the component of greatest percentage, the methaneis the component of second greatest percentage, and nitrogen is presentin a percentage substantially less than methane.

Other objects and features of the present invention will appear from thefollowing detailed description considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagramillustrating one presently preferred closed cycle refrigeration systememploying a multicomponent refrigerant in accordance with the invention;and

FIG. 2 is a simplified schematic diagram illustrating the major flowcircuits comprising a complete liquefaction facility or plantconstructed in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION With reference to FIG. 1 of thedrawings, the natural gas liquefaction cycle includes a heat exchangedevice 10 which is vertically disposed as illustrated with its warm end11 being its lower end and its cold end 12 being its upper end. The heatexchange device includes an outer shell 13 defining an elongated shellspace 14 or zone within which is positioned a plurality of coiled tubesor other means forming separate passageways for flow of fluids inout-of-contact heat interchange with fluid flowing through the shellspace to provide an elongated heat exchange passageway 15, a firstseries of heat exchange passageways 16, 17, 18, and 119 and a secondseries of heat exchange passageways 20, 21, 22, and 23. The passageway15 extends throughout the length of the shell space 14 and is connectedto a natural gas inlet conduit 24 at the warm end 111 and to a liquefiednatural gas outlet conduit 25 at the cold end 12. The series ofpassageways 16, 17, 18, and 19 and the series of passageways 20, 21, 22,and 23 are vertically positioned in the shell space with pairs ofpassageways 16 and 20, 17 and 21, I8 and 22, and 19 and 23, disposedbetween similar temperature levels along the heat exchange zone. Theheat exchange device also includes a plurality of liquid distributormeans 26, 27, 28, 29, and 30, provided with feed conduits 31, 32, 33,34, and 35, respectively, located at spaced vertical positions in theshell space. The liquid distributor means 26, 27, 28, and 29 are locatedat or slightly above the pairs of passageways 16 and 20, 17 and 21, 18and 22, and 19 and 23, respectively, and function to distribute liquiddirectly onto respective pairs of passageways as well as directly ontothe portion of the elongated passageway 15 adjacent respective pairs ofpassageways. The liquid distributor means 30 is located at the cold end12 and functions to distribute liquid directly onto the cold end of thepassageway 15.

The cycle also includes a compressor 36, an aftercooler 37 and aplurality of phase separators 38, 39, 40, and 41. The inlet of thecompressor is connected by conduit 42 to the shell space 14 at the warmend of the heat exchange device 10 and its outlet is connected byconduit 43 to passage 414 of the aftercooler in heat interchange withcoolant fluid, such as water,

flowing through the shell space 45, and then by conduit 46 to the phaseseparator 38. The phase separators 38, 39, 40, and 41 are provided withvapor outlet conduits 47, 48, 49, and 50, respectively, and with liquidoutlet conduits 51, 52, S3, and 54, respectively; the vapor outletconduits 47, 48, 49, and 50 being respectively connected to the warmends of passageways 16, 17, 18, and 19 and the liquid outlet conduits51, 52, 53, and 54 being respectively connected to the warm ends ofpassageways 20, 21, 22, and 23. The phase separators 39, 40 and 41 arealso provided with feed conduits 55, 56 and 57, respectively, and thefeed conduits are connected to the cold ends of passageways 16, 17 and18, respectively; the cold end of the passageway 19 being connected byconduit 58 and pressure reducing valve 59 to the feed conduit 35 forliquid distributor means 30. The liquid distributor means 26, 27, 28,and 29 are connected through their feed conduits to the cold ends ofpassageways 20, 21, 22, and 23; in particular, the passageway 20 isconnected by conduit 60 and pressure reducing valve 61 to the feedconduit 31, the passageway 21 is connected by conduit 62 and pressurereducing valve 63 to feed conduit 32, the passageway 22 is connected byconduit 64 and pressure reducing valve 65 to feed conduit 33, and thepassageway 23 is connected by conduit 66 and pressure reducing valve 67to feed conduit 34.

The natural gas to be liquefied, previously purified and freed ofmoisture, enters the cycle under pressure at ambient temperature throughconduit 24 and upon flowing through the passageway 15 is graduallycooled to liquefaction temperature and is withdrawn from the cycle byconduit 25 totally in liquid phase and preferably subcooled forsubsequent pressure reduction as required for storage or other use. Ifdesired, the natural gas feed before leaving the passageway 15 may bereduced in pressure to an intermediate superatmospheric pressure andflowed at such pressure through the cold end portion of passageway 15located above the liquid distributor means 29. in situations where thenatural gas includes high boiling point components which are not desiredin the liquid product, the natural gas feed, after initial cooling toeffect liquefaction of undesirable high boiling point components, may bewithdrawn from the passageway 15 by conduit 68 and fed to a phaseseparator 69 from which the liquefied high boiling point components arewithdrawn by conduit 70 and the remaining unliquefied natural gasreturned by conduit 71 to the passageway 15 for continued flow throughthe heat exchange device 10. If desired, the natural gas may bewithdrawn from the passageway 15 at more than one temperature level toeffect plural separation of undesired high boiling point components.

Refrigeration for the process is provided by a closed refrigerationsystem employing a multicomponent refrigerant, i.e., a refrigerantcomprising a mixture of elemental gases having different boiling points.The multicomponent refrigerant is successively fractionated by partialcondensation in a plurality of steps to provide liquids at progressivelydecreasing temperature levels and such liquids are introduced underreduced pressure into the zone of the heat exchange device atcorresponding temperature levels to provide the refrigeration requiredto effect liquefaction and subcooling of the natural gas feed as well asthe refrigeration required to effect the fractional condensation of themulticomponent refrigerant. The multicomponent refrigerant is deliveredby the compressor 36 under superatmospheric pressure and the heatinterchange in the aftercooler 37 effects the first fractionalcondensation step at the highest temperature level, the resulting liquidvapor mixture being separated in the phase separator 38 to provide thefirst condensate 72. The second fractional condensation step is effectedby passing unliquefied refrigerant from the phase separator 38 throughthe passageway 16 to effect its partial condensation followed byseparation in the phase separator 39 to provide the second condensate73. In a similar manner, third and fourth fractional condensation stepsare performed by passing unliquefied refrigerant withdrawn from thephase separators 39 and 60 through the passageways l7 and 18,respectively, to effect partial liquefaction with the respectiverespective liquefied portions being separated in phase separators 40 and41 providing a third condensate 74 and the fourth condensate 75. Theunliquefied refrigerant withdrawn from the phase separator 41 by conduit50 is cooled upon flowing through the passageway 19 to effect at leastits partial condensation and, in some operations, the fluid leaving thepassageway 19 by conduit 58 may be totally in liquid phase. In eitherevent, liquid in the conduit 58 comprises a fifth condensate. The lattercondensate and the condensates 72, 73, 74, and 75 have differentcompositions of the components that comprise the multicomponentrefrigerant fed to the compressor 36, the first condensate 72 being richin high boiling point components, the last condensate in conduit 58being rich in low boiling point components and the intermediatecondensates 73, 74, and 75 having decreasing high boiling pointcomponents and increasing low boiling point components in the ordernamed. The condensates are formed under uniform pressure, except forpressure drops resulting from friction of the conduits; however, theexact composition of each condensate will depend upon the composition ofthe multicomponent refrigerant fed to the inlet of compressor 36 andupon the temperature of the liquid-vapor mixtures leaving thepassageways 16, 17, 18, and 19. In any event, however, the bubble pointtemperature of the condensates 72, 73, 74, and 75 and the condensate inconduit 58 will progressively decrease in the order named which is theorder of progressively increasing composition of low boiling componentsof the multicomponent refrigerant. Again, depending upon the componentsand their percentage composition of the multicomponent refrigerant fedto the compressor 36, the composition of the condensates will vary notonly with respect to percentage composition of components but also withrespect to presence of components. For example, the condensate inconduit 58 having the lowest bubble point temperature will contain ahigh percentage of low boiling point components and will not contain thehighest boiling point components. It is to be understood that the numberof fractional condensation steps and hence the number of condensates ofthe multicomponent refrigerant may vary. As a specific example, thecubiole shown in the drawing may be modified to include three phaseseparators, such as phase separators 38, 39 and 41 with passageways 18and 22 removed and the passageways 16, 17, 19, 20, 21, and 23 extendedas required to provide heat exchange passageways extending substantiallythroughout the zone between the cold end of the passageway 19 and thewarm end of the passageway 16.

The condensates are reduced in pressure by valves 61, 63, 65, 67, and 59to pressures existing in the shell space 14 in the regions of respectiveliquid distributor means 26, 27, 28, 29, and 30. Such pressures willcorrespond, except for variations due to the pressure gradient along theheat exchange device 10, to the inlet pressure of the compressor 36which is preferably above atmospheric pressure. The condensates 72, 73,74, and 75, prior to pressure reduction, are subcooled upon flowingthrough passageways 20, 21, 22, and 23, respectively; however, in somecycles, the subcooling step may be employed. The condensates underreduced pressure are fed by conduits 31, 32, 33, 34, and 35 torespective liquid distributor means 26, 27, 28, 29, and 30, each locatedin the shell space 14 where the temperature therein correspondssubstantially to the temperature of the respective condensates underreduced pressure. The condensate from the distributor means 30 isvaporized upon heat interchange with relatively warm fluid flowingthrough the upper part, as viewed in the drawing, of the passageway 15and the condensates from the distributor means 26, 27, 28, and 29 arevaporized by heat interchange with relatively warm fluid in thepassageway 15 and by heat interchange with relatively warm fluidsflowing through the pairs of passageways 16 and 20, 17 and 21, 18 and22, and 19 and 23, respectively. The condensates, being mixtures ofcomponents ofdifferent boiling points, will vaporize within a range ofincreasing temperatures and the vapor from all of the condensates willflow through the shell space 14, in a direction from the cold end 12 tothe warm end 11, and leave the heat exchange device through thecompressor feed conduit 42. it will thus be appreciated that therefrigerant, in liquid and vapor phases, flows through the vapor space14 in countercurrent heat interchange with the natural gas feed in thepassageway 15 and with portions of the refrigerant in passageways 16,17, 18, 19, 20, 21, 22, and 2.3 to effect cooling, liquefaction andsubcooling of the natural gas feed and the plural fractionalcondensation steps to provide the condensates and to effect subcoolingof the condensates.

The efficiency of a multicomponent refrigeration cycle as describedabove depends upon a large number of parameters which are complexlyinterrelated; certain of such parameters depend directly while othersdepend indirectly on the composition of the refrigerant entering thesuction inlet of the compressor 36 through the conduit 42; suchrefrigerant is the multicomponent refrigerant of the process and theterm multicomponent refrigerant used herein and in the appended claimsrefers to the composition of the mixture of gases as exists whencompressed from the relatively low vaporization pressure to therelatively high condensation pressure. In order to cool the liquefiednatural gas to desired subcooled temperature, it is necessary that thecondensate discharged from the liquid distributor means 30 be at asufficiently lower temperature. Thus, the multicomponent refrigerantmust include a component having a boiling point temperature below thesubcooled temperature of the feed mixture and, in addition, the quantityof such component in the multicomponent refrigerant must be such as toprovide the proper composition in the condensate from the liquiddistributor 30 after the preceding fractional condensation steps, toattain the required low temperature. Also, the composition of thecondensates 72, 73, 74, and 75 will depend upon the composition of themulticomponent refrigerant, the exhaust pressure of the compressor 36and the temperature at which the fractional condensation occurs, and thelatter temperatures will likewise depend upon the composition of themultiple component refrigerant, and also upon the inlet pressure of thecompressor 36, that is, the vaporization pressure of the condensates,and a relationship involving the quantity of available refrigerationutilized in cooling the feed stream. Furthermore, insofar as compressionefficiency is concerned, the inlet pressure and exhaust pressure of thecompressor 36 may be advantageously set at specific superatmosphericpressures. in addition, the overall efficiency of the process dependsupon the heat interchange efficiency between the natural gas feedflowing through the passageway 15 and the multicomponent refrigerantflowing in the shell space 14 in countercurrent heat interchangetherewith. in order to obtain optimum heat exchange efficiency, it isnecessary to establish and maintain between the natural gas feed and themulticomponent refrigerant a specific temperature differenceproportional to the absolute temperature level. The temperaturedifference between the countercurrently flowing fluids in the passageway15 and the shell space 14 will depend upon a number of factors whichinclude the composition of the multicomponent refrigerant, that is, thespecific components of the refrigerant and the percentage composition ofsuch components.

it has been determined that a natural gas liquefaction process using amulticomponent refrigeration as described above may be operated withoptimum efficiency but employing a multicomponent refrigerant consistingof specific components in specific percentage relationships bysatisfying the optimum requirements of parameters of the processincluding optimum heat interchange between the natural gas feed and themulticomponent refrigerant. in accordance with the principles of thepresent invention, a novel multicomponent refrigerant which achieves theforegoing results is broadly characterized to include the following:

1. A mixture of component gases having different boiling pointtemperatures including nitrogen, methane and hydrocarbons having two ormore carbon atoms.

2. The component having the third lowest boiling point temperaturecomprising the component of greatest percentage of the mixture.

3. The component having the second lowest boiling point temperaturecomprising the component of second greatest percentage of the mixture.

The foregoing broad characteristics and other distinguishingcharacteristics of the multicomponent refrigerants provided by thepresent invention have been embodied in the actual design ofmulticomponent refrigeration systems for effecting liquefaction ofnatural gas of varying compositions as demonstrated by the specificexamples appearing hereinafter.

FIG. 2 is a simplified schematic diagram illustrating the major flowcircuits comprising a complete liquefaction facility or plant. Referringfirst to the upper left-hand portion of FIG. 2 of the drawings, thenatural gas feed is supplied to the liquefaction plant through apipeline as a two-phase mixture having a major portion in the gaseousphase and a minor portion in the liquid phase. This feed mixture isinitially separated in a separator 112 from which the minor portion iswithdrawn at the bottom as a liquid and pressurized by a liquid pump114. The major portion is withdrawn in the gaseous phase from the top ofthe drum, compressed in compressor 1 16 and heat exchanged with coolingwater in exchanger 118. The two portions of the feed are then joined andexpanded into a flash drum wherein a portion of the original gaseousfeed is liquefied and mixes with that portion of the feed which wasinitially in the liquid phase. The total liquid comprising most of theheavy hydrocarbons (i.e., heavier than C hydrocarbons) is then withdrawnfrom the bottom of flash drum 120 and supplied through a line 122 to aconventional fractionation plant 123, the general operation of whichwill be described hereinafter although the particular details of theplant form no part of the present invention. The gaseous fraction of thefeed comprising most of the methane, nitrogen and the C to Chydrocarbons is withdrawn from the top of flash drum 120 and passedthrough one or more conventional absorbers 124 which remove impuritiessuch as hydrogen sulfide and carbon dioxide. The gaseous feed is thenslightly cooled by heat exchange with cooling water in exchanger 126 andpassed through line 128 to the lowermost heat exchanger coil 130 locatedin the bottom of main heat exchanger 132 wherein the stream issufficiently cooled so that the water and most of the C and Chydrocarbons are condensed and separated out in the first stage 134 of atwo-stage separator 136. The major portion of the stream is withdrawn inthe gaseous phase through line 138 and is passed through one or moredriers 141) wherein the remaining water is removed. After drying, themain stream flows through line 142 to intermediate heat exchange coils144 wherein the stream is further cooled to form a second liquidfraction which is separated in the second stage 146 of separator 136;this fraction containing most of the C and C hydrocarbons. The majorportion of the main stream remains in the gaseous phase and is conductedthrough lines 150 and 151 to low temperature coil 152 of the mainexchanger 132 wherein the feed stream is totally liquefied. In order toprevent vapor losses during subsequent expansion of the LNG toatmospheric pressure, the pressure of the liquefied natural gas isreduced from 660 to 206 p.s.i.a. by passage through expansion valve 15dprior to passage through exchanger coil 156 wherein the LNG issubcooled. Thus, the provision of valve 1554 enables the enthalpy of theLNG to be reduced so that the liquid is not vaporized during the finalpressure letdown in passing through expansion valve 158 after which theliquid is maintained at atmospheric pressure in storage tank 159.

From the foregoing description of the main process stream, it isapparent that all of the refrigeration required to liquefy and subcoolthe feed stream (except for the very small amount of refrigerationprovided by water coolers 118 and 126) is provided by main exchanger132. This exchanger is an integral unit composed of a plurality ofcylindrical shell segments 166, 162, 164 and 166 connected by aplurality of frustoconical thin-H: mun

transition sections 168, 170, and 172. in addition to the feed streamcoils previously mentioned, the exchanger includes a plurality ofrefrigerant coils 174, 176, 178, and 180 as well as a plurality ofrefrigerant spray headers 182, 18 3, 186, 188, and 190.

The above-mentioned refrigerant coils and spray headers, together with amultistage refrigerant separator 192 and a refrigerant compressor 194-,form the entire refrigeration system which will now be described indetail beginning with compressor inlet line 196 shown in the bottomright-hand corner of FIG. 2 of the drawings. Line 196 contains a singlegaseous refrigerant which is a mixture of a plurality of component gaseshereinafter referred to as a multicomponent refrigerant (MCR). Forexample, a preferred multicomponent refrigerant consists by volume of 31parts methane, 35 parts ethane, 7 parts propane, 14 parts butane, 4parts pentane, 3 parts hexane, and 6 parts nitrogen. This multicomponentrefrigerant is compressed in stage A of compressor 194 and cooled ininterstage water cooler 197 so that a portion is condensed and thenseparated in separator 198, The condensate is withdrawn from the bottomof the separator and pumped directly into the first stage 199 of MCRseparator 192. The gaseous fraction of the refrigerant is withdrawn fromthe top of separator 198, compressed in stage B, cooled in water cooler200 and joined with the previously mentioned condensate which issupplied to separator 1.92 at a pressure in the order of l 5 p.s.i.a.and at a temperature of 100 F.

in the first stage 199 of MCR separator 192, the liquid fraction rich inC and heavier hydrocarbons is separated and supplied through line 201and pressure reduction valve 202 to spray header 182, from which it issprayed downwardly over the lower portions of coil 152 as well as coils130, 144, and 174 whereby the liquid refrigerant is vaporized in coolingthe fluids in the coils.

Referring back to separator 192, the gaseous fraction is withdrawn fromstage 199 through line 203, cooled in coil 174 and returned to thesecond stage 204 of the separator wherein a second fraction of liquefiedrefrigerant is separated. This liquid fraction at a temperature in theorder of 17 F. is rich in the C, to C hydrocarbons and is suppliedthrough line and pressure reduction valve 207 to spray header 184 whichis positioned above coil 176 and at an intermediate point along coil152. Thus, this second liquid fraction of the refrigerant is sprayedover coils 152 and 176 whereby the refrigerant is vaporized in coolingthe fluids in these coils.

The gaseous fraction in stage 204 is withdrawn through line 208, cooledin coil 176, and returned to the third stage 210 of the separatorwherein a third fraction of liquefied refrigerant is separated. Thisliquid fraction at a temperature in the order of minus 71 F. is rich inthe C, to C hydrocarbons and is supplied through line 212 and pressurereduction valve 213 to spray header 186 which is positioned above coil178 and at a point near the upper portion of coil 152. Thus, thisfraction of the liquid refrigerant is sprayed over coil 178 and theintermediate portion of coil 152 whereby the refrigerant is vaporized incooling the fluids in these coils.

The gaseous fraction in stage 210 of the separator is withdrawn throughline 214, cooled in coil 178 and returned to the fourth stage 216 of theseparator wherein a fourth liquid fraction is separated. This liquidfraction at a temperature in the order of minus 140 F. is rich innitrogen and the C, to C hydrocarbons and is supplied through line 218and pressure reduction valve 219 to spray header 188 which is positionedabove coils 152 and 180. Thus, the fourth liquid fraction is vaporizedin cooling the feed in the upper end of coil 152 as well as the lastremaining fraction of the refrigerant which is supplied to coil 160through line 220 from stage 216. This last fraction of the refrigerantrich in nitrogen and methane is liquefied in passing through coil 180from which it exits at a temperature in the order of minus 206 F. and isreduced in pressure by passage through expansion valve 222 whereby itstemperature drops to minus 259 F. Thereafter, it is supplied to sprayheader 1% which is positioned above subcooling coil 156 so that the lastrefrigerant fraction is vaporized in sub cooling the feed stream in coil156 to a temperature in the order of minus 258 F.

From the foregoing description, it is apparent that each of the liquidfractions of the multicomponent refrigerant is vaporized by heatexchange with the feed stream and high pressure refrigerant fractions atspecific temperature levels. For example, the temperature levels in thevicinity of spray headers 182, 184, 186, 188, and may be in the order of9 F., minus 79 F., minus 146 F., minus 228 F., and minus 259 F.,respectively. At the same time, the temperatures of the MCR fractionsdownstream of pressure reduction valves 202, 207, 213, 219, and 222 arein the order of 27 F., minus 66 F., minus 142 F., minus 220 F., andminus 259 F.

After being vaporized in heat exchanger 132, each of the MCR fractionsare recombined, withdrawn through line 224 at a pressure in the order of41 p.s.i.a. and recycled back to compressor 194 along with a smallamount of makeup refrigerant which is supplied through line 226. Thismakeup refrigerant, as well as the original charge of refrigerant, isobtained from the feed stream except for the nitrogen which is suppliedfrom an air separation plant 228 through control valve 229. That is, theliquid fractions in stages 134 and 146 of feed separator 136 arewithdrawn through lines 230 and 232, dried in drier 234, and supplied tothe previously mentioned fractionation plant 123 through line 236. Thisplant is conventional in that it consists of a plurality offractionation columns which separate the natural gas feed from line 236into the components. Thus, predetermined amounts of ethane, propane,butane, pentane, and hexane are withdrawn through lines 238, 240, 242,244, and 246 as determined by control valves 248 while methane is addedto makeup line 226 through branch line 250 and control valve 252. inorder to maintain a desired heating value of the LNG, controlled amountsof the C, to C hydrocarbons are withdrawn from the fractionation plantthrough line 254 and added to main process stream in line 150.

From the foregoing description of the liquefaction plant of FIG. 2 ofthe drawings, it will be apparent that significant economies in initialcapital investment are possible due to the fact that the utilization ofa single refrigerant requires a single compressor as opposed to theutilization of separate refrigerants in cascade wherein each refrigerantrequires a separate compressor. In addition, the effective utilizationof more than three hydrocarbons plus nitrogen substantially reduces thecompression horsepower since closer matching of the cooling curves ispossible at each temperature level in exchanger. Furthermore,substantial functional as well as economic advantages are obtained fromthe utilization of a one-piece, multitemperature level exchanger asopposed to a plurality of individual exchangers operating overindividual temperature ranges which cannot be matched so exactly to theoptimum cooling curve of the feed stream. Significant advantages in thecost and ease of fabrication also flow from the combination of utilizingmore than three hydrocarbons as an MCR refrigerant in an integralexchanger in that at least the majority if not all of the spray headersmay be physically positioned between adjacent MCR coils as opposed totheir physical location intermediate the inlet and outlets of the coils.Lastly, the utilization of pressure reduction valve 154 reduces theundesired flash losses of the liquefied product while the utilization ofinterstage phase separator 198 decreases operating horsepowerrequirements. Of course, it is to be understood that the foregoingdescription is intended to be illustrative rather than exhaustive of theinvention and that the latter is not to be limited other than asexpressly set forth in the claims including all patentable equivalentsthereof.

The following specific examples further illustrates the invention.

lllllllfi llllfll ExA'MTi Lm A multicomponent refrigeration process ofthe type described herein for liquefying natural gas of the followingcomposition:

Component Mol Percent by Volume Nitrogen l.l2 Methane 70.02 C, 15.23 c,8.06 10 c, 3.80 c, L28 u 0.49

The natural gas feed, freed of hydrogen sulfide, carbon dioxide andwater is introduced into the process at about 100 F. and under apressure of about 620 p.s.i.a. in a heat exchange zone in countercurrentheat interchange with a multicom' ponent refrigerant fractionated bypartial condensation in three successive steps. After initial cooling toeffect liquefac' tion of high boiling point components and afterseparation of such components, natural gas feed of the followingcomposition is flowed through the heat exchange zone:

Component Mol Percent by Volume Nitrogen 1.15

Methane 71.03

Before leaving the heat exchange zone, the natural gas feed was reducedin pressure to about 150 p.s.i.a. and flowed from the zone totally inliquid phase at a temperature of about 263 F. for subsequent storageunder a pressure of about 15 p.s.i.a. and a temperature of about 262.5F. The multicomponent refrigerant entered the suction of the compressorat about p.s.i.a. and was discharged from the compressor at about 450p.s.i.a. The multicomponent refrigerant entering the compressorconsisted of the following mixture having an average mol'ecular weightof 34.94:

Component Mol Percent by Volume Nitrogen 0.8l

Methane 88.20

The natural gas feed, free of hydrogen sulfide, carbon dioxide and wateris introduced into the process at about F. and under a pressure of about620 p.s.i.a. in a heat exchange zone in countercurrent heat interchangewith a multicomponent refrigerant fractionated by partial condensationin three successive steps. After initial cooling to effect liquefactionof high boiling point components and after separation of suchcomponents, natural gas feed of the following composition is flowedthrough the heat exchange zone:

Component Mol Percent by Volume Nitrogen 0.83 Methane 9015 C, 4.19 2.74c, 1.67 C, 0.32 u 0.10

Before leaving the heat exchange zone, the natural gas feed was reducedin pressure to about 150 p.s.i.a. and flowed from the zone totally inliquid phase at a temperature of about 263 F. for subsequent storageunder a pressure of about 15 p.s.i.a. and a temperature of about -262.5F. The multicom ponent refrigerant entered the suction of the compressorat about 40 p.s.i.a. and was discharged from the compressor at about 450p.s.i.a. The multicomponent refrigerant entering the compressorconsisted of the following mixture having an average molecular weight of33.42:

Component Mol Percent by Volume Nitrogen 4.85 Methane 32.50 C, 315.50 c,6.55 C, 8.50 C, l0.80 C 0.30

EXAMPLE III A multicomponent refrigeration process of the type describedherein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.7] Methane 89.l3 C, 5.68 c2.46 C, 1.1 1 C 0.49 C, 0.42

The natural gas feed, free of hydrogen sulfide and carbon dioxide, isintroduced into the process at about 100 F. and under a pressure ofabout 650 p.s.i.a. in a heat exchange zone in countercurrent heatinterchange with a multicomponent refrigerant fractionated by partialcondensation in three successive steps. After initial cooling to effectliquefaction of high boiling point components and after separation ofsuch components, natural gas feed of the following composition if flowedthrough the heat exchange zone:

Component Mol Percent by Volume Nitrogen 0,74 Methane 93.2l 5.19 C; 0.72C4 0.10 5. 0.04

Before leaving the heat exchange zone, the natural gas feed was reducedin pressure to about p.s.i.a. and flowed from the zone totally in liquidphase at a temperature of about 264 F. for subsequent storage under apressure of about 15 p.s.i.a. and a temperature of about 262.5 F. Themulticomponent refrigerant entered the suction of the compressor atabout 42 p.s.i.a. and was discharged from the compressor at about 450p.s.i.a. The multicomponent refrigerant entering the compressorconsisted of the following mixture having an average molecular weight of34.28:

Component Mol Percent by Volume Nitrogen 4.40 Methane 27.40 c, 41.00 C,5.50 c, 12.50 c, 9.20

EXAMPLE IV A multicomponent refrigeration process of the type describedherein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.43 Methane 99.49 c, 0.06 c0.01 C C,. C 0.01

The natural gas feed, free of hydrogen sulfide, carbon dioxide andwater, is introduced into the process at about 60 F. and under apressure of about 600 p.s.i.a. in a heat exchange zone in countercurrentheat interchange with a multicomponent refrigerant fractionated bypartial condensation in three successive steps. The natural gas feedflows from the zone totally in liquid phase under a pressure of about550 p.s.i.a. and at a temperature of about 263 F. The multicomponentrefrigerant entered the suction of the compressor at about 50 p.s.i.a.and was discharged from the compressor at about 465 p.s.i.a. Themulticomponent refrigerant entering the compressor consisted of thefollowing mixture having an average molecular weight of 34.49:

Component Mol Percent by Volume Nitrogen 5.42 Methane 22.50 C, 41.70 C11.92 C. 12.43 C 6.03

EXAMPLE V A multicomponent refrigeration process of the type describedherein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 1.18 Methane 66.98 C, 17.22 c,8.94 4.03 c, 1.27 c. 0.38

The natural gas feed, free of hydrogen sulfide, carbon dioxide andwater, is introduced into the process at about 100 F. and under apressure of about 615 p.s.i.a. in a heat exchange zone in countercurrentheat interchange with a multicomponent refrigerant fractionated bypartial condensation in three successive steps. After initial cooling toeffect liquefaction of high boiling point components and afterseparation of such components, natural gas feed of the followingcomposition is flowed through the heat exchange zone:

Component Mol Percent by Volume Nitrogen 1.20 Methane 68.34

Before leaving the heat exchange zone, the natural gas feed was reducedin pressure to about 150 p.s.i.a. and flowed from the zone totally inliquid phase at a temperature of about 263 F. for subsequent storageunder a pressure of about 15 p.s.i.a. and a temperature of about 262.5F. The multicomponent refrigerant entered the suction of the compressorat about 40 p.s.i.a. and was discharged from the compressor at about 450p.s.i.a. The multicomponent refrigerant entering the compressorconsisted of the following mixture having an average molecular weight of34.73:

undo

Component Mol Percent by Volume Nitrogen 4.85 Methane 27.80 C, 38.00 c6.00 C, 14.50 C, 8.55 C 0.30

EXAMPLE VI A multicomponent refrigeration process of the type describedherein for liquefying natural gas of the following composition:

Component M01 Percent by Volume Nitrogen 0.36 Methane 99.54 C, 0.10

The natural gas feed, free of hydrogen sulfide, carbon dioxide andwater, is introduced into the process at about F. and under a pressureof about 615 p.s.i.a. in a heat exchange zone in countercurrent heatinterchange with a multicomponent refrigerant fractionated by partialcondensation in three successive steps. Before leaving the heat exchangezone, the natural gas feed was reduced in pressure to about p.s.i.a. andflowed from the zone totally in liquid phase at a temperature of about263 F. for subsequent storage under a pressure of about 15 p.s.i.a. anda temperature of about 262.5 F. The multicomponent refrigerant enteredthe suction of the compressor at about 40 p.s.i.a. and was dischargedfrom the compressor at about 450 p.s.i.a. The multicomponent refrigerantentering the compressor consisted of the following mixture having anaverage molecular weight of 3 l .03:

Component Mol Percent by Volume Nitrogen 5.0 Methane 32.6 C, 43.11 C,2.0 C 1 1.0 c, 5.0

EXAMPLE Vll A multicomponent refrigeration process of the type describedherein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.36 Methane 99.54 C, 0.10

The natural gas feed, free of hydrogen sulfide, carbon dioxide andwater, is introduced into the process at about 100 F. and under apressure of about 620 p.s.i.a. in a heat exchange zone in countercurrentheat interchange with a multicomponent refrigerant fractionated bypartial condensation in three successive steps. Before leaving the heatexchange zone, the natural gas feed was reduced in pressure to about 150p.s.i.a. and flowed from the zone totally in liquid phase at atemperature of about 263 F. for subsequent storage under a pressure ofabout 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponentrefrigerant entered the suction of the compressor at about 40 p.s.i.a.and was discharged from the compressor at about 450 p.s.i.a. Themulticomponent refrigerant entering the compressor consisted of thefollowing mixture having an average molecular weight of 33.17:

Component Mol Percent by Volume Nitrogen 5.5 Methane 24.0 C, 43.5 C,12.0 C, 10.0 C 50 EXAMPLE Vlll A multicomponent refrigeration process ofthe type described herein for liquefying natural gas of the followingcomposition:

Component Mol Percent by Volume The natural gas feed, free of hydrogensulfide, carbon dioxide and water, is introduced into the process atabout 100 F. and under a pressure of about 615 p.s.i.a. in a heatexchange zone in countercurrent heat interchange with a multicomponentrefrigerant fractionated by partial condensation in three successivesteps. The natural gas feed was reduced in pressure to about 150p.s.i.a. and flowed from the zone totally in liquid phase at atemperature of about 263 F. for subsequent storage under a pressure ofabout 15 p.s.i.a. and a temperature of about 262.5 F. The multicomponentrefrigerant entered the suction of the compressor at about 40 p.s.i.a.and was discharged from the compressor at about 450 p.s.i.a. Themulticomponent refrigerant entering the compressor consisted of thefollowing mixture having an average molecular weight of 34.45:

Component Mol Percent by Volume Nitrogen 7.0 Methane 29.5 C: 37.5 C; 3.6C, 9.0 C, l3.4

EXAMPLE IX A multicomponent refrigeration process of the type describedherein for liquefying natural gas of the following composition:

Component Mol Percent by Volume Nitrogen 0.11 Methane 90.33 1 5.44 C2.03 C, 1.30 c, 0.46 C, 0.33

The natural gas feed, free of hydrogen sulfide, carbon dioxide andwater, is introduced into the process at about 100 F. and under apressure of about 620 p.s.i.a. in a heat exchange zone in countercurrentheat interchange with a multicomponent refrigerant fractionated bypartial condensation in three successive steps. The natural gas feed wasreduced in pressure to about 150 p.s.i.a. and flowed from the zonetotally in liquid phase at a temperature of about 263 F. for subsequentstorage under a pressure of about 15 p.s.i.a. and a temperature of about262.5 F. The multicomponent refrigerant entered the suction of thecompressor at about 40 p.s.i.a. and was discharged from the compressorat about 450 p.s.i.a. The multicomponent refrigerant entering thecompressor consisted of the following mixture having an averagemolecular weight of 34.03:

From the foregoing examples, it will be appreciated that eachmulticomponent refrigerant disclosed comprises a mixture of componentgases including nitrogen, methane and hydrocarbons having two or morecarbon atoms; that nitrogen is the component of lowest boiling pointtemperature; that the hydrocarbon having two carbon atoms, the componenthaving the third lowest boiling point temperature, comprises thecomponent of greatest percentage of the mixture and that methane, thecomponent having the second lowest boiling point temperature, comprisesthe component of second greatest percentage of the mixture. It will befurther appreciated from the foregoing examples that, in multicomponentrefrigerants including hydrocarbons having three, four and five carbonatoms, the component comprising the third greatest percentage of themixture is selected from the group consisting of a hydrocarbon havingthree carbon atoms, a hydrocarbon having four carbon atoms and ahydrocarbon having five carbon atoms. It has been determined that, byusing multicomponent refrigerants of compositions provided by thepresent invention, natural gas may be liquefied with a higher overallefficiency as compared to use of multicomponent refrigerants ofcompositions provided by the prior art. It is believed that theincreased efficiency results at least in part by the provision of amulticomponent refrigerant in which the component having the thirdlowest boiling point temperature comprises the component of greatestpercentage of the mixture; such component being less than 50 percent ofthe mixture and preferably within a range of about 35 percent to 45percent of the mixture. Another characteristic of the multicomponentrefrigerants provided by the present invention not found in priormulticomponent refrigerants is the feature that the components of thegreatest percentage and the second greatest percentage of the mixturetogether make up more than 50 percent of the mixture, preferably fallingwithin a range of about 64 percent to about 77 percent of the mixture.The percentage composition of the methane and hydrocarbon having twocarbon atoms in multicomponent refrigerants including heavierhydrocarbons provides mixtures 0 having average molecular weights whichfall within a range of about 31 to about 35. In this regard, it will beappreciated that natural gas, a multicomponent refrigerant provided bythe prior art, usually has an average molecular weight of 20 or less butin any event does not exceed 25. Of the natural gas feeds in theexamples, only the natural gas feeds of Examples 1 and V have averagemolecular weights in excess of 20, Le, 23.23 and 23.77, respectively.Also, the multicomponent refrigerant suggested by Kleemenko, i.e., 65percent methane, 20 percent propane and 15 percent butane, has anaverage molecular weight of 27.96. Furthermore, it will be appreciatedfrom the foregoing examples that, in addition to the multicomponentrefrigerants having the foregoing characteristics, the present inventionalso provides multicomponent refrigerants having additionalcharacteristics depending upon the component which makes up the thirdgreatest percentage of the mixture. When the component of the thirdgreatest percentage of the mixture comprises a hydrocarbon having threecarbon atoms, a hydrocarbon having four carbon atoms comprises thecomponent of fourth greatest percentage of the mixture and nitrogencomprises the component of fifth greatest percentage of the mixture.Also, when the component of the third greatest percentage of the mixtureis a hydrocarbon having four carbon atoms, the component of the fourthgreatest percentage of the mixture is selected from a group consistingof a hydrocarbon having three carbon atoms and a hydrocarbon having fivecarbon atoms; when the component of the fourth greatest percentage is ahydrocarbon having three carbon atoms, the component of the fifthgreatest percentage comprises a hydrocarbon having five carbon atomswhereas, when the component of the fourth greatest percentage of themixture is a hydrocarbon having five carbon atoms, the component of thefifth greatest percentage of the mixture is selected from a groupconsisting of nitrogen and a hydrocarbon having three carbon atoms. lnaddition, when the component of the third greatest percentage is ahydrocarbon having five carbon atoms, the component of the fourthgreatest percentage of the mixture is a hydrocarbon having four carbonatoms and the component of the fifth greatest percentage of the mixtureis selected from a group consisting of nitrogen and a hydrocarbon havingthree carbon atoms.

There is thus provided by the present invention multicomponentrefrigerants of novel composition adapted particularly for use inliquefying natural gas of varying composition. While the multicomponentrefrigerants vary with respect to the percentage of the components, eachembodies the discovery that the component of greatest percentage of themixture comprises the component having the third lowest boiling point,Le, a hydrocarbon having two carbon atoms, e.g., ethane and/or ethylene,and that the component of next greatest percentage of the mixturecomprises methane, the component having the second lowest boiling point.From the foregoing examples, it will be appreciated that, in addition tothe variations discussed above, the percentage of the mixture comprisingthe hydrocarbon having two carbon atoms may vary within the range ofabout 35 percent to about 45 percent and that percentage of the methanein the mixture may vary within the range of about 22 percent to 36percent. Reference therefore will be had to the appended claims for adefinition of the limits of the invention.

We claim:

1. A method of liquefying a feed stream composed primarily of methanecomprising:

a. compressing a multicomponent refrigerant comprising a plurality ofindividual hydrocarbon components having different boiling points,

b. said plurality of hydrocarbon components including a C hydrocarbonselected from the group of ethane and ethylene as the component ofgreatest percentage, and methane as the component of second greatestpercentage,

c. said multicomponent refrigerant further including nitrogen comprisingat least 4 percent by volume of the multicomponent mixture.

d. progressively condensing and phase separating said compressedmulticomponent refrigerant into a plurality of liquid condensates andvapor portions of progressively colder temperatures,

. expanding each of said progressively colder condensates,

vaporizing each of said progressively colder condensates in heatexchange with portions of said feed stream and separated vapor portionsof said multicomponent refrigerant so as to progressively condense saidmulticomponent refrigerant according to step (d) while progressivelyliquefying said feed stream, said vaporizations of said progressivelycolder condensates against said feed stream portions and saidmulticomponent vapor portions occurring at the same vaporizationpressure for each condensate,

g. withdrawing said vaporized condensates from said heat exchangermeans, and

h. recycling said withdrawn vaporized condensates as said multicomponentrefrigerant in a closed cycle to a compressor for compressing the samein accordance with step a.

2. The method as claimed in claim 1 wherein the C hydrocarbon is presentin an amount constituting at least 35 percent by volume of saidmulticomponent refrigerant.

3. The method as claimed in claim 1 wherein the average molecular weightof the multicomponent refrigerant is equal to or greater than 30.

4. A system for liquefying a feed stream composed primarily of methanecomprising:

a. compressor means for compressing a multicomponent refrigerantcomprising a plurality of hydrocarbons including a C hydrocarbonselected from the group of ethane and ethylene as the component ofgreatest percentage,

and further including substantially more than 3 percent by volume of anonhydrocarbon component having a normal boiling point substantiallybelow that of methane,

b. a plurality of heat exchange means and phase separator meansconnected in series for progressively condensing and separating saidcompressed multicomponent refrigerant into a plurality of liquidcondensates and vapor portions decreasing temperatures,

c. expansion means for expanding each of said liquid condensates,

d. multistage heat exchanger means for vaporizing each of said expandedliquid condensates at the same vaporization pressure in each stagethereof, each of said heat exchanger stages including a shell portionand heat exchange passage means within each respective shell portion forpassing one of said expanded liquid condensates in heat exchangerelationship with both said feed stream and at least one of said vaporportions separated from said unexpanded condensate of saidmulticomponent refrigerant and undergoing condensation, and

. closed cycle means for withdrawing all of said vaporized condensatesfrom said heat exchanger means and passing the same as saidmulticomponent refrigerant to said compressor means.

5. The system as claimed in claim 4 wherein the C hydrocarbon is presentin an amount constituting at least 35 percent by volume of saidmulticomponent refrigerant.

6. The system as claimed in claim 4 wherein the average molecular weightof the multicomponent refrigerant is equal to or greater than 30.

7. The system as claimed in claim 4 wherein the combined percentage ofthe nonhydrocarbon component plus the C and heavier hydrocarboncomponents is greater than 11 percent but less than 21 percent byvolume.

8. The method as claimed in claim 1 wherein the combined percentage ofthe nitrogen component plus the C and heavier hydrocarbon components isgreater than 11 percent but less than 21 percent by volume.

2%? TED STATES PATENT o TtT cRTTTTcE tcTTN Patent No. 3.6%,106 a dFebruary 29, 1972 Inventm-( Lee S. Gaumer, Jr. and Charles L. Newton Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In Line 9 of the Abstract after "gas" insert --to be liquefied to effecteooling; and liquefaction of the natural gas-- In the Specification:

Column 1, line 7, "preset" should read -present--;

' and "out" should read --our--.

Column 1, line 9, "out" should read --our-- Column 1, line 11,"Refrigerator" should read --Refrigerant-- Column 1, line 13, "Seru No722,136" should read Ser, No. 722,135-- 1 Column 5, line 6 4, "butshould read --by-- Signed and sealed this 26th day of September 1972.

(SEAL) Attest:

EDWARD M..FLETCHER,JR. ROBERT GOTTSCHALK ilttesting; OfficerCommissioner of Patents-

2. The method as claimed in claim 1 wherein the C2 hydrocarbon ispresent in an amount constituting at least 35 percent by volume of saidmulticomponent refrigerant.
 3. The method as claimed in claim 1 whereinthe average molecular weight of the multicomponent refrigerant is equalto or greater than
 30. 4. A system for liquefying a feed stream composedprimarily of methane comprising: a. compressor means for compressing amulticomponent refrigerant comprising a plurality of hydrocarbonsincluding a C2 hydrocarbon selected from the group of ethane andethylene as the component of greatest percentage, and further includingsubstantially more than 3 percent by volume of a nonhydrocarboncomponent having a normal boiling point substantially below that ofmethane, b. a plurAlity of heat exchange means and phase separator meansconnected in series for progressively condensing and separating saidcompressed multicomponent refrigerant into a plurality of liquidcondensates and vapor portions decreasing temperatures, c. expansionmeans for expanding each of said liquid condensates, d. multistage heatexchanger means for vaporizing each of said expanded liquid condensatesat the same vaporization pressure in each stage thereof, each of saidheat exchanger stages including a shell portion and heat exchangepassage means within each respective shell portion for passing one ofsaid expanded liquid condensates in heat exchange relationship with bothsaid feed stream and at least one of said vapor portions separated fromsaid unexpanded condensate of said multicomponent refrigerant andundergoing condensation, and e. closed cycle means for withdrawing allof said vaporized condensates from said heat exchanger means and passingthe same as said multicomponent refrigerant to said compressor means. 5.The system as claimed in claim 4 wherein the C2 hydrocarbon is presentin an amount constituting at least 35 percent by volume of saidmulticomponent refrigerant.
 6. The system as claimed in claim 4 whereinthe average molecular weight of the multicomponent refrigerant is equalto or greater than
 30. 7. The system as claimed in claim 4 wherein thecombined percentage of the nonhydrocarbon component plus the C5 andheavier hydrocarbon components is greater than 11 percent but less than21 percent by volume.
 8. The method as claimed in claim 1 wherein thecombined percentage of the nitrogen component plus the C5 and heavierhydrocarbon components is greater than 11 percent but less than 21percent by volume.